1. Field of the Invention
The invention relates to a high-pressure hose composed of a plastic core and a plurality of layers of helically wound pressure-resisting members, and the method for the manufacture of same, the members formed in particular of wires of high tensile strength which are rewound in a rewinding device from a supply reel onto several working spools, and optionally, of an outer cover made of a plastic material.
High-pressure hoses should possess good flexibility and bending properties coupled with some partial elasticity even at high operating pressures. To obtain these properties, a number of production techniques have come into use and will now be outlined.
2. Description of the Prior Art
In the manufacture of hose suited for high-pressure applications, and in particular of hose made of vulcanizable elastomers with braided or coiled pressure-resisting members, it is known to apply vulcanizable adhesion promotors/adhesives between the outer wall of the core and the pressure-resisting member as well as between the individual pressure-resisting members or between pressure-resisting members and the outer cover for the purpose of improving the adhesion. The bonding layer formed during vulcanization then is part of the hose wall. Apart from the advantage of improved homogeneity and the attendant higher fatigue strength and ability to withstand high working pressures, a good "rubber-to-metal" bond is achieved, and when the hose is cut to the desired lengths there is no unraveling of the braiding or of the coiled metal insert.
Moreover, with regard to such hose it is known to condition metal wires to improve their affinity for other adhesion promoters by electroplating them either with brass or with other metals. This is how their "affinity for rubber", alluded to above, in the vulcanizing process is obtained. An important prerequisite, however, is that the steel wires used have high grease resistance. In practice, this is very difficult to secure. For improvement of the rubber-to-metal bond, it is further known to add heavy-metal oxides to the bonding agent, which may produce a further improvement in the breaking-load values and hence in flexibility.
The measures described in German Pat. No. 1,188,388 relate solely to hose made of elastomers. In the case of the hose described in German Pat. No. 2,014,887, the use of vulcanizable mixtures as an intermediate layer is proposed for improvement of the flexibility.
Many other publications describe measures intended to improve the homogeneity and to secure the pressure-resisting members in the hose wall in order to prevent unraveling when the hose is cut to length. In all these publications, the assumption is made that vulcanization of the prefabricated hose is essential.
As is known, vulcanization is dispensed with in the production of thermoplastic pressure-resisting members, and in particular of coiled steel-wire pressure-resisting members. An inner tube and outer cover are strictly extruded. The conventional use of precoiled steel wires makes it unnecessary to use a separate adhesive since the coiled wires or windings remain in their original position even without the use of a separate adhesive.
Thermoplastic hose, and especially thermoplastic hose of higher compressive strength, requires the use of thermoplastics having relatively high strength properties. Now such materials are necessarily more rigid, and this is true especially of the inner core. In use, they therefore exhibit relatively poor bending properties, especially when more than two steel-wire windings must be applied. This is the case particularly with hose types which are designed for very high pressures, have a thicker core wall, and are made of plastics of the polyamide type, for example.
In the case of thermoplastic hose, too, it is known to apply adhesives between every two oppositely directed windings for the purpose of improving the flexibility. Since there is no vulcanization, these adhesives usually contain solvents. They are applied between the core of the hose and the first winding as well as between individual windings.
The drawback of such measures in the manufacture of thermoplastic hose is that the solvents contained in the adhesive must be eliminated before the outer cover is applied so that no gaseous release of residual solvents occurs due to the heat as the outer cover is extruded on. Such "outgassing" would preclude bonding of the outer cover to the pressure-resisting members. It is further possible to use two-component adhesives in place of solvent-containing adhesives, or to apply thermoplastics as bonding agents by the use of an additional extruder directly in the stranding operation. However, all these expedients require additional equipment which complicates handling considerably and entails extra costs.
Insertion of a hot-melt adhesive foil tape between the individual pressure-resisting layers, as proposed in German Pat. No. 2,223,523, has also proved deleterious in volume production since such foil must be activated through a heat treatment in an additional operation and thus entails extra costs.
In the manufacture of pressure hose made of elastomers and comprising steel-wire layers as pressure-resisting members, it is known to coil the steel wires in a forming device while production of the hose is in progress in order that the individual wires, adapted to the diameter of the hose core used, may be wrapped around the latter more or less firmly.
It is known to perform this coiling operation of the individual wires in the stranding machine, in other words, as they are wound onto the particular hose layer. This wire-forming operation thus is carried out just ahead of the so-called winding point by guiding the wires over a rounded sharp edge, whereby they are coiled.
At least two important requirements must be met to achieve such coiling. In the first place, the wire must have a certain minimum tension between the working spool and the winding point. Secondly, the hose core to be reinforced must possess sufficient compressive strength. This can be secured by the use of so-called mandrels which prior to their introduction into the helical-winding machine are supercooled to the point where they have very high rigidity.
However, this technique has several drawbacks: The changing diameter of the wire on the spool necessarily results in varying tensile forces, which steadily increase as the diameter of the coil of wire on the spool decreases in the course of production and therefore give rise to changing spiraling forces. Adjustment of the spool braking forces is difficult or possible only at very great cost since as is known the spools are arranged in a circle on a revolving stranding disk. The brakes of the spool must be firmly preset manually before production gets under way. In actual practice, this has a detrimental effect on precise maintenance of the hose diameter.